the nation's top doctors are sounding an alarm about something they're seeing coast to coast. And here is the map that says it all. Every region marked in red is facing an outbreak of the flu. And there is no flu season that has started this hard, this early in a decade. So, why is this year so bad and what about the flu shot? Abc's chief medical editor dr. Richard besser starts us off. Reporter: It's an early outbreak. In arizona, this time last year, there were 18 cases of flu. Today, 790. In new york, 84 last year. Today, 3,975. Massachusetts, 126 last year. Today, we are seeing an average of over 600 cases a week of infl influenza like illness. Reporter: From kentucky to north carolina, to texas. Horrible sore throat and ears. Just makes you feel bald for five, seven days. It's nasty. Reporter: Hospitals now seeing the influx. We are having an early influenza season and it's a serious season. We've had a definite uptick in hospitalizations. Reporter: There hasn't been an outbreak this early for ten years. And that year, the flu season was severe. We know that with just one sneeze the virus can spread almost 20 feet in just seconds. You are infectious a full day before you show any symptoms, a bad mix. But why would this flu season, which usually peaks in february, be spreading so quickly so soon? Is this a new flu, one that isn't in this year's sack seine, one we're not immune to? So far, the vaccine seems to be a good match. I can't remember it starting early in november. Reporter: Or more intriguing, could dry air be the culprit? It is the most interesting new theory. In damp air, the water droplets take the flu to the ground. Dry air? Look at that sneeze again, the virus floats in the air longer spreading further. All doctors know right now is that the flu is already here. So, rich besser is here right now. Tell me more about the vaccine this year, how effective is it? Reporter: It's never 100%. Usually 60% in healthy adults, lower in the elderly who need it. That's why we all need to get the vaccine, so we don't infect them. But we keep hearing over and over again from people, I got the flu shot and I was more vulnerable afterwards to the flu. Possible? Reporter: It just can't happen. You may get a sore arm and a fever, but you are not getting the flu. So, get the flu shot, no matter what. Reporter: It's better than nothing. All right, rich besser reporting in on this outbreak

This transcript has been automatically generated and may not be 100% accurate.

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